src/Pure/tactic.ML

 
 (*Makes a rule by applying a tactic to an existing rule*)
 fun rule_by_tactic tac rl =
   let
     val ctxt = Variable.thm_context rl;
-    val ([st], ctxt') = Variable.import true [rl] ctxt;
+    val ((_, [st]), ctxt') = Variable.import true [rl] ctxt;
   in
     (case Seq.pull (tac st) of
       NONE => raise THM ("rule_by_tactic", 0, [rl])
     | SOME (st', _) => zero_var_indexes (singleton (Variable.export ctxt' ctxt) st'))
   end;